Methods for heat treating and manufacturing a thermomechanical part made of a titanium alloy, and thermomechanical part resulting from these methods

ABSTRACT

A method of heat treating a thermomechanical part made of a TA6Zr4DE titanium alloy. A solution-annealing is performed at a temperature lying in the range beta transus −20° C. and beta transus −15° C. for a duration lying in the range four hours to eight hours. The method is applicable to fabricating high pressure compressor disks.

The invention relates to a method of heat treating a thermomechanical part made of a TA6Zr4DE titanium alloy, to a fabrication method including such a heat treatment method, and to a thermomechanical part that results from those methods.

The invention relates more particularly, but not exclusively, to rotary parts for turbomachines, such as disks, trunions, and impellers, and in particular to high pressure compressor disks.

At present, in the technique used by the Applicant, high pressure compressor disks are obtained by stamping the titanium alloy in the beta domain. In particular, it is preferred to use an alloy such as “6242” that includes about 6% aluminum, 2% tin, 4% zirconium, and 2% molybdenum. More precisely, the alloy concerned is known as TA6Zr4DE in metallurgical nomenclature. The stamping is performed at about 1030° C.

The stamping step is followed by a heat treatment method comprising a step of solution annealing in the alpha/beta domain of the alloy at a temperature of 970° C. for one hour, where 970° C. is 30° C. lower than to the beta transus temperature, i.e. corresponds to the beta transus temperature −30° C. This solution-annealing step is followed by a step of quenching in oil or in a water-polymer mixture. Thereafter, tempering treatment is performed at 595° C. for eight hours, and finally cooling is performed in air.

When implementing that heat treatment method, an alloy is obtained that presents a coarse microstructure that is not favorable to obtaining good strength for the titanium alloy, in particular when subjected to a creep test under an imposed stress that is maintained for a certain holding time, in particular for a range of utilization temperatures of −50° C. to +25° C. It is the “dwell effect”, i.e. creep at fairly low temperature (lower than 200° C.), that leads to damage which, in association with oligocyclic fatigue, leads to premature failure of the part.

In particular, the application in the aviation field, and in particular for a high pressure compressor disk, is very propitious for this “dwell effect” phenomenon since during the stages of take-off and landing engines are subjected to operating conditions in the temperature and stress domain that corresponds to this phenomenon. This phenomenon can lead to premature starting of fatigue cracks, or even to the parts breaking.

This “dwell effect” phenomenon is very well identified by the manufacturers of turbine engines and it has been the subject of a large amount of study; furthermore, it applies to all temperature-stabilized titanium alloys: titanium alloys in the beta, alpha/beta, near-alpha, and alpha classes.

An object of the present invention is to provide a method of heat treating a thermomechanical part made of a titanium alloy, which method can be implemented industrially and serves to overcome the drawbacks of the prior art, and in particular makes it possible to limit the extent of the “dwell effect” phenomenon.

It is therefore desirable to improve the heat treatment so as to obtain parts having a longer lifetime, in spite of being subjected to cyclical stressing at low temperature.

To this end, according to the present invention, the heat treatment method is characterized in that a solution-annealing step is performed at a temperature lying in the range beta transus −20° C. and beta transus −15° C. for a duration lying in the range four hours to eight hours.

This temperature condition corresponds to a maximum temperature of about 985° C. This difference relative to the beta transus temperature constitutes a safety margin, associated with the possibility of a difference between the temperature as measured and the real temperature of the alloy, and it serves to guarantee that the alloy remains below its beta transition temperature. This solution-annealing step is performed for four to eight hours depending on the size of the part.

The idea on which the present invention is based corresponds to the fact that it has been observed that within the material there are to be found zones or colonies that are propitious for the “dwell effect” phenomenon. It is observed that such colonies are formed by elongate needle-type alpha phase grains that are relatively fat and that touch one another. In general, such grains present a length of several millimeters and a width of the order of 200 micrometers (μm) to 300 mm. When stresses accumulate, such colonies constitute locations in which a large number of dislocations become concentrated such that, on becoming active, and without any particular thermal effect, they can cause slip to take place between the grains, which can lead to breakage.

The present invention proposes implementing heat treatment that enables the microstructure to be refined, in particular by refining the size of the above-mentioned needle, so as to minimize the effects of the “dwell effect”, and in particular by diminishing the extent to which dislocations can move freely so as to minimize the extent to which they accumulate, thereby minimizing any risk of the part breaking.

That is why, in a manner characteristic of the present invention, the solution-annealing step is performed for a duration that is much longer than is usual. In this way, the part is allowed to come close to or even to reach its microstructural equilibrium, thereby enabling the needles in the colonies that might give rise to the “dwell effect” to be reduced in size both in length and in thickness. This treatment enables a microstructure to be obtained that is finer than that in the prior art, and thus serves to minimize the consequences of the “dwell effect”.

Surprisingly, this increase in the duration of the solution-annealing step does not have the consequence of spoiling the thermomechanical properties of the material, contrary to current prejudices in the field of metallurgy. In quite surprising manner, in the context of the invention described herein, the inventors have implemented a heat treatment method in which the solution-annealing step is performed for a duration that is much longer than the usual duration, but without the material that results from the heat treatment method as a whole presenting thermomechanical characteristics, and in particular properties of fatigue strength under imposed stress, that are weaker than those of the materials that result from the prior art heat treatment method.

In addition, the present invention proposes performing this solution-annealing step at a temperature that is quite close to the beta transition temperature, while remaining strictly below said temperature, in order to obtain a microstructure for the final part that lies in the alpha/beta, near-alpha, and alpha classes.

In this way, it can be understood that merely by lengthening the duration of the solution-annealing step, it is possible to obtain thermomechanical parts, in particular high pressure compressor disks, presenting firstly lifetimes that are longer than those of parts obtained using previous techniques, but also presenting thermomechanical characteristics (strength in traction, in creep, in fatigue under imposed stress with holding time . . . ) that are at least as good, and while minimizing the risks of fatigue failure. Thus, the heat treatment method of the invention makes it possible to improve the ability to withstand the “dwell effect” by a factor of about two compared with a prior art heat treatment method, as shown by testing described below, where the “dwell effect” involves cyclical loading, with the loading being held for a certain length of time on each cycle to encourage creep.

Advantageously, after the solution-annealing step, the method in accordance with the invention further includes a step whereby the part is quenched at a cooling rate greater than 200° C./min, and preferably lying in the range 300° C./min to 450° C./min. Preferably, this cooling rate is as high as possible and is preferably greater than or of the order of 400° C./min.

Thus, because of this fast cooling, the state of the microstructure is frozen in the situation in which it is to be found at the end of the long solution-annealing step, thereby avoiding any new variation in this microstructure leading to growth of the needles in the alpha phase colonies, which growth would be propitious to the “dwell effect” phenomenon.

Thus, this choice of a high quenching speed serves to encourage a martensitic type transformation of the beta phase into the alpha phase (and giving rise to a fairly fine microstructure), as compared with the germination/growth type phenomenon (which leads to a microstructure that is rather coarse).

Also preferably, at the end of the heat treatment method in accordance with the invention, the method further includes the following steps:

-   -   after the quenching step, a tempering step at a temperature of         about 595° C. for a duration of about eight hours; and then

a step of cooling in air.

In addition to the above-described heat treatment method, the invention also provides a method of fabricating a thermomechanical part made of a titanium alloy, by stamping in the beta domain, which fabrication method includes such a heat treatment method.

The present invention also provides a thermomechanical part made of a titanium alloy in which the fabrication method includes the above-mentioned heat treatment method or results from the above-described fabrication method.

The titanium thermomechanical part preferably forms a rotary part of a turbine engine, in particular a compressor disk, specifically for a high pressure compressor.

Finally, the present invention also provides a turbomachine fitted with a thermomechanical part according to any of the definitions given above.

Other advantages and characteristics of the invention appear on reading the following description made by way of example and with reference to the accompanying drawing, in which:

FIG. 1 shows the microstructure obtained using the conventional heat treatment method of the prior art;

FIG. 2 shows the microstructure obtained using the conventional heat treatment method of the prior art as modified by a faster quenching speed;

FIG. 3 shows the microstructure obtained using the heat treatment method of the present invention;

FIG. 4 shows the microstructure obtained using the heat treatment method of the present invention with a faster quenching speed; and

FIG. 5 shows the results of a creep test under cyclic loading with a load holding time for a part obtained by the prior art method and for a part obtained by the method in accordance with the invention.

It is recalled that the present invention relates to all types of temperature-stabilized titanium alloy: titanium alloys in classes beta, alpha/beta, near-alpha, and alpha (with these terms relating to the structure of the finished part).

It is recalled that the conventional heat treatment used in particular by the Applicant for high pressure compressor disks made of an alloy known as “6242” and mentioned in the introduction consists in the following.

The disks are obtained by forging using hot stamping in the beta domain of the titanium alloy.

This stamping step is followed by a method of heat treatment comprising a step of solution annealing in the alpha/beta domain of the alloy at a temperature of 970° C., i.e. 30° C. lower than the beta transus temperature, and for one hour. This solution-annealing step is followed by a step of quenching in oil or in a water-polymer mixture (cooling rate of the order of 200° C./min and lying in the range 130° C./min to 250° C./min). Thereafter, a tempering operation is performed at 595° C. for eight hours, and finally cooling is performed in air.

That produces a material presenting the microstructure that can be seen in FIG. 1, presenting colonies constituted by mutually parallel needles of beta phase. Those needles present a section of elongate shape that is visible in the figure, and often extending over several hundreds of micrometers.

In FIG. 2, the microstructure that can be seen corresponds to that of a titanium alloy identical to the alloy of FIG. 1, having been subjected to the above-described heat treatment with the exception of the following two differences:

-   -   the solution annealing temperature was beta transus −20° C.         (about 980° C.) instead of beta transus −30° C.; and     -   the quenching speed used during the heat treatment method was         considerably faster: 400° C./min instead of 200° C./min, e.g.         using water quenching instead of oil quenching, and after taking         care to avoid excessive thicknesses of material, possibly by         prior machining of the thickest zones.

Under such circumstances, the colonies of parallel needles comprise needles that are more dissimilar in size, and in particular there are fewer large needles. Nevertheless, even though less numerous, it can be expected that there will be sufficient of these large needles for the “dwell effect” phenomenon to lead to accumulations of dislocations that are liable to give rise to risks of breakage.

With reference to FIGS. 3 and 4, there can be seen the microstructures that are obtained by using the method in accordance with the present invention.

More precisely, compared with the conventional heat treatment method described above with reference to FIG. 1, the treatment implemented to achieve the microstructure of FIG. 3 was as follows:

-   -   solution annealing at the beta transus temperature −20° C.         (about 980° C.) instead of at beta transus −30° C.; and     -   the solution annealing was performed for eight hours instead of         one hour.

Under such circumstances, and as can be seen in FIG. 3, the needles are all of smaller section size, of length that remains less than 100 μm, and is generally about 50 μm.

It can thus be understood that the reduction in the size of the needles is accompanied by a reduction in their volume and in a reduction of the areas of joint between needles, thereby putting a brake on the ability to move of defects such as dislocations or vacancies, so they move over shorter distances and have less chance of accumulating.

In FIG. 4, compared with the heat treatment performed on the alloy shown in FIG. 3, quenching was also performed at a faster rate, at 400° C./min instead of 200° C./min.

The idea was thus to increase the quenching speed to above the value of 200° C./min, and if possible to approach the value of 400° C./min. Nevertheless, it is necessary to avoid cooling too fast, since that runs the risk of causing quench cracks to appear. In particular, at faster than 450° C./min, there is a risk of inducing stresses that prevent any subsequent machining, or even that run the risk of breaking the part.

In terms of microstructure, and as can be seen in FIG. 4, the result is similar to that of FIG. 3.

Thus, it can be seen that increasing the quenching speed and/or lengthening the duration of the solution-annealing step serves to diminish the damage to the material that results from cyclical stressing, which damage is another factor of failure of the material, conventionally occurring in addition to damage by creep.

More precisely, by means of these treatment modifications, the microstructures are frozen to a greater extent at a size that is smaller than the size of microstructures that give rise to damage in the material. This avoids needles or grains accumulating in the form of bunches of large-sized parallel needles that, like a single grain, concentrate defects at the edges of their interfaces.

Thus, by reducing both the size of the colonies made up of bunches of parallel needles and the size of the needles themselves, more obstacles are created against defects, and in particular against vacancies, progressing and thus potentially grouping together.

Samples coming firstly from materials obtained using the heat treatment method of the prior art with the microstructure of FIG. 1, and secondly from materials obtained using the heat treatment method of the present invention and having the microstructure of FIG. 3 have been tested in creep.

More precisely, a test was performed under cyclic loading including a load holding time by implementing a cycle of trapezoid shape: increasing load for 1 second (s), load maintained constant for 120 s at 868 megapascals (MPa), followed by load dropping down to zero over 1 s.

The results of that test are shown in FIG. 5, which is a graph showing the ratio of deformation over lengthening under cyclic loading with a load holding time, plotted as a function of the number of cycles to breakage.

Curve A shows the result of that test for materials obtained using the heat treatment method of the prior art and having the microstructure of FIG. 1.

Curve B shows the result of that test for materials obtained using the heat treatment method of the present invention and having the microstructure of FIG. 4.

That standardized test thus shows that the heat treatment method of the present invention makes it possible practically to double the number of cycles before breaking, since the number is raised from 5500 cycles to 10,000 cycles.

Thus, surprisingly, the present invention makes it possible, in particular by lengthening the duration of the solution-annealing step, significantly to improve lifetime during a fatigue strength test that includes a load-holding time. This is due mainly to the fact that the lengthening of the duration serves to refine the microstructure, and in particular to reduce the size of the alpha-phase needles that form the colonies that are sensitive to the “dwell effect” phenomenon.

In practice, for large parts that do not allow fast quenching speeds to be used, longer solution-annealing times are selected (e.g. eight hours), and for finer parts where it is possible to reach a quenching speed of 400° C./min, shorter solution-annealing times can be applied (e.g. four hours).

Furthermore, it is known that increasing the solution annealing temperature encourages the coarse primary alpha phase to dissolve in order to be transformed into beta phase. Nevertheless, since it is fundamental not to exceed the beta transus temperature of the alloy, a temperature should be selected that does not exceed the beta transus temperature −15° C. This upper limit on the solution annealing temperature is selected depending on the accuracy with which the beta transition temperature is known and the class of oven used for the treatment. Furthermore, when performing sub-transus forging, i.e. at a temperature below the beta transition temperature, it is naturally appropriate to select a solution-annealing temperature that is higher than the forging temperature.

Other tests (traction, creep, fatigue with holding time under maximum stress, . . . ) for measuring the mechanical strength of the materials obtained by the method of the invention have confirmed that overall they conserve their mechanical properties compared with the titanium alloys obtained by the conventional method, i.e. these results remain within the statistical mean of the results obtained for analogous parts that were subjected to heat treatment, but without modification in accordance with the present invention. 

1-9. (canceled)
 10. A method of heat treatment for a thermomechanical part made out of a TA6Zr4DE titanium alloy, the method comprising: a solution-annealing at a temperature lying in the range beta transus −20° C. and beta transus −15° C. for a duration lying in the range four hours to eight hours.
 11. A heat treatment according to claim 10, further comprising: after the solution-annealing, quenching the part at a cooling rate greater than 200° C./min.
 12. A heat treatment method according to claim 11, wherein the cooling rate, during the quenching the part, lies in the range 300° C./min to 450° C./min.
 13. A heat treatment method according to claim 12, further comprising: after the quenching, a tempering at a temperature of about 595° C. for a duration of about eight hours; and then cooling in air.
 14. A method of fabricating a thermomechanical part made of a titanium alloy, the fabrication being by stamping in the beta domain, and the method comprising: a heat treatment method according to claim
 10. 15. A thermomechanical part made of a titanium alloy in which the fabrication method includes a heat treatment method according to claim
 10. 16. A thermomechanical part according to claim 15, forming a rotary part of a turbomachine.
 17. A thermomechanical part according to claim 15, forming a high pressure compressor disk.
 18. A turbomachine including a thermomechanical part according to claim
 15. 